Generally, liquid fuel such as gasoline or diesel oil is supplied to an internal combustion engine by injecting the liquid fuel to a suction path or a combustion chamber from a fuel injection valve. A pressure of a space in which the liquid fuel is to be injected is lower than a pressure of the liquid fuel since the liquid fuel is pressurized by a fuel pump. Hence, the injected liquid fuel is flash boiled so that the injected fuel is atomized. However, the injected fuel cannot be atomized sufficiently when the temperature of the liquid fuel and the temperature of the space to which the liquid fuel is to be injected are low, for example, at cold start of the internal combustion engine, since the liquid fuel might not be flash boiled.
Hence, in some conventional internal combustion engines, a heater is provided so that, when the internal combustion engine is at a low temperature, the liquid fuel is heated before being supplied to an ultrasound injection valve, which atomizes the fuel by ultrasonic wave. One of such a conventional internal engine is disclosed in Japanese Utility Model Laid-Open No. H05-061446, according to which a particulate contained in exhaust gas is suppressed by ultrasonically atomizing the heated liquid fuel.